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# Transformer

Member Posts: 3
I know this realy doesn't go with the theme of the board, but I hope one of you could help me with this. I need a way to convert 12v DC out of a car battery to 120v Ac so I can use a home radio and amp for a parade. I know they make these converters but they are not to cheep just for 2 parades. So I was wandering why can't I just use a transformer from Radio Shack, they are only around \$15, I was just wandering what I should get. On the web it says 120VAC Primary, and 12.5 Secondary. Would that work? I am not sure if I could just reverse it and how would I convert the DC input to AC output?

Thanks for any help,
Paul

• Member Posts: 93
: I know this realy doesn't go with the theme of the board, but I hope one of you could help me with this. I need a way to convert 12v DC out of a car battery to 120v Ac so I can use a home radio and amp for a parade. I know they make these converters but they are not to cheep just for 2 parades. So I was wandering why can't I just use a transformer from Radio Shack, they are only around \$15, I was just wandering what I should get. On the web it says 120VAC Primary, and 12.5 Secondary. Would that work? I am not sure if I could just reverse it and how would I convert the DC input to AC output?
:
:
: Thanks for any help,
: Paul
:
Actually Paul, this really does go with the theme of the board! A nice idea too, but unfortunately it won't work. You can hook up a Radio Shack transformer "backwards" to get 110VAC out by putting 12VAC in. BUT, it would have to be AC, not the DC from a car battery.

This is why power inverters are expensive -- they must have circuitry to convert the DC to AC so that a transformer can be used to increase the voltage. Most inverters us transistors as automatic switches to convert the DC to AC. A special transformer is used that has extra windings on it to tell the transistors when to "switch". You can use simple transformers in simple circuits, but they won't work quite as well. Here is one simple circuit:

http://www.i4at.org/lib2/inverter.htm

Note that the transformer must be 24v -- two 12v windings together -- one 12v winding for the negative half of the AC, and one 12v winding for the positive half.

Building a good power inverter is a moderately complicated project.

Enjoy!

rg
• Member Posts: 198
: : I know this realy doesn't go with the theme of the board, but I hope one of you could help me with this. I need a way to convert 12v DC out of a car battery to 120v Ac so I can use a home radio and amp for a parade. I know they make these converters but they are not to cheep just for 2 parades. So I was wandering why can't I just use a transformer from Radio Shack, they are only around \$15, I was just wandering what I should get. On the web it says 120VAC Primary, and 12.5 Secondary. Would that work? I am not sure if I could just reverse it and how would I convert the DC input to AC output?
: :
: :
: : Thanks for any help,
: : Paul
: :
: Actually Paul, this really does go with the theme of the board! A nice idea too, but unfortunately it won't work. You can hook up a Radio Shack transformer "backwards" to get 110VAC out by putting 12VAC in. BUT, it would have to be AC, not the DC from a car battery.
:
: This is why power inverters are expensive -- they must have circuitry to convert the DC to AC so that a transformer can be used to increase the voltage. Most inverters us transistors as automatic switches to convert the DC to AC. A special transformer is used that has extra windings on it to tell the transistors when to "switch". You can use simple transformers in simple circuits, but they won't work quite as well. Here is one simple circuit:
:
: http://www.i4at.org/lib2/inverter.htm
:
: Note that the transformer must be 24v -- two 12v windings together -- one 12v winding for the negative half of the AC, and one 12v winding for the positive half.
:
: Building a good power inverter is a moderately complicated project.
:
: Enjoy!
:
: rg
:
Also, that 120vac and 12vac are rms, and with a simple oscillator, you'll be getting 120vpeak, which is significantly higher. I'm not sure if that circuit solves that problem, and it doesn't give the ratio of the transformer. Also note that if you do use that inverter, you will be supplying an inductive load, as the power supply of the home 120vac radio probably uses a transformer and rectifies and filters the output for a nice dc voltage for the amplifier to use.
• Member Posts: 3
Where would be the best place to find a 24V, Center Tapped Transformer?? How much can I expect to pay for one???

thanks,
Paul
• Member Posts: 198
: Where would be the best place to find a 24V, Center Tapped Transformer?? How much can I expect to pay for one???
:
:
: thanks,
: Paul
:

I like digikey.com
• Member Posts: 93
: : : I know this realy doesn't go with the theme of the board, but I hope one of you could help me with this. I need a way to convert 12v DC out of a car battery to 120v Ac so I can use a home radio and amp for a parade. I know they make these converters but they are not to cheep just for 2 parades. So I was wandering why can't I just use a transformer from Radio Shack, they are only around \$15, I was just wandering what I should get. On the web it says 120VAC Primary, and 12.5 Secondary. Would that work? I am not sure if I could just reverse it and how would I convert the DC input to AC output?
: : :
: : :
: : : Thanks for any help,
: : : Paul
: : :
: : Actually Paul, this really does go with the theme of the board! A nice idea too, but unfortunately it won't work. You can hook up a Radio Shack transformer "backwards" to get 110VAC out by putting 12VAC in. BUT, it would have to be AC, not the DC from a car battery.
: :
: : This is why power inverters are expensive -- they must have circuitry to convert the DC to AC so that a transformer can be used to increase the voltage. Most inverters us transistors as automatic switches to convert the DC to AC. A special transformer is used that has extra windings on it to tell the transistors when to "switch". You can use simple transformers in simple circuits, but they won't work quite as well. Here is one simple circuit:
: :
: : http://www.i4at.org/lib2/inverter.htm
: :
: : Note that the transformer must be 24v -- two 12v windings together -- one 12v winding for the negative half of the AC, and one 12v winding for the positive half.
: :
: : Building a good power inverter is a moderately complicated project.
: :
: : Enjoy!
: :
: : rg
: :
: Also, that 120vac and 12vac are rms, and with a simple oscillator, you'll be getting 120vpeak, which is significantly higher. I'm not sure if that circuit solves that problem, and it doesn't give the ratio of the transformer. Also note that if you do use that inverter, you will be supplying an inductive load, as the power supply of the home 120vac radio probably uses a transformer and rectifies and filters the output for a nice dc voltage for the amplifier to use.
:
Kor, you bring up some interesting issues.

Yes, there will be a difference in performance of a device that is powered by an inverter compared to being powered by the mains. The output waveform of the inverter will be more-or-less a square wave with at about 120 volts peak voltage. Since the RMS voltage of a square wave is the same as its peak voltage, the RMS output will also be about 120 volts. This differs from the mains sine wave which has a peak voltage of 1.414 times the RMS voltage, or about 170 volts peak.

As a result, the power supply capacitors in the amplifier / device under inverter power will not be able to charge to as high a voltage, so the internal DC power supply voltage will be slightly lower than normal. In practice, this is usually not a problem.

You are right about inductive loads Kor; they can be a problem. Hopefully, the rectifiers / capacitors / regulator / load in the device under power will represent a more-or-less resistive load and that load will be reflected through it's power transformer. But actually, the load will likely be somewhat inductive, and this will cause voltage spikes in the inverter which can zap the invertor transistors. In the schematic on the web site, the components R1/D2/R2/D1 form a network to protect the transistors from voltage spikes.

Hope you are all staying cool, the temp almost made it to 100 deg f here today in northern California.

Enjoy!

rg
• Member Posts: 198
: : : : I know this realy doesn't go with the theme of the board, but I hope one of you could help me with this. I need a way to convert 12v DC out of a car battery to 120v Ac so I can use a home radio and amp for a parade. I know they make these converters but they are not to cheep just for 2 parades. So I was wandering why can't I just use a transformer from Radio Shack, they are only around \$15, I was just wandering what I should get. On the web it says 120VAC Primary, and 12.5 Secondary. Would that work? I am not sure if I could just reverse it and how would I convert the DC input to AC output?
: : : :
: : : :
: : : : Thanks for any help,
: : : : Paul
: : : :
: : : Actually Paul, this really does go with the theme of the board! A nice idea too, but unfortunately it won't work. You can hook up a Radio Shack transformer "backwards" to get 110VAC out by putting 12VAC in. BUT, it would have to be AC, not the DC from a car battery.
: : :
: : : This is why power inverters are expensive -- they must have circuitry to convert the DC to AC so that a transformer can be used to increase the voltage. Most inverters us transistors as automatic switches to convert the DC to AC. A special transformer is used that has extra windings on it to tell the transistors when to "switch". You can use simple transformers in simple circuits, but they won't work quite as well. Here is one simple circuit:
: : :
: : : http://www.i4at.org/lib2/inverter.htm
: : :
: : : Note that the transformer must be 24v -- two 12v windings together -- one 12v winding for the negative half of the AC, and one 12v winding for the positive half.
: : :
: : : Building a good power inverter is a moderately complicated project.
: : :
: : : Enjoy!
: : :
: : : rg
: : :
: : Also, that 120vac and 12vac are rms, and with a simple oscillator, you'll be getting 120vpeak, which is significantly higher. I'm not sure if that circuit solves that problem, and it doesn't give the ratio of the transformer. Also note that if you do use that inverter, you will be supplying an inductive load, as the power supply of the home 120vac radio probably uses a transformer and rectifies and filters the output for a nice dc voltage for the amplifier to use.
: :
: Kor, you bring up some interesting issues.
:
: Yes, there will be a difference in performance of a device that is powered by an inverter compared to being powered by the mains. The output waveform of the inverter will be more-or-less a square wave with at about 120 volts peak voltage. Since the RMS voltage of a square wave is the same as its peak voltage, the RMS output will also be about 120 volts. This differs from the mains sine wave which has a peak voltage of 1.414 times the RMS voltage, or about 170 volts peak.
:
: As a result, the power supply capacitors in the amplifier / device under inverter power will not be able to charge to as high a voltage, so the internal DC power supply voltage will be slightly lower than normal. In practice, this is usually not a problem.
:
: You are right about inductive loads Kor; they can be a problem. Hopefully, the rectifiers / capacitors / regulator / load in the device under power will represent a more-or-less resistive load and that load will be reflected through it's power transformer. But actually, the load will likely be somewhat inductive, and this will cause voltage spikes in the inverter which can zap the invertor transistors. In the schematic on the web site, the components R1/D2/R2/D1 form a network to protect the transistors from voltage spikes.
:
: Hope you are all staying cool, the temp almost made it to 100 deg f here today in northern California.
:
: Enjoy!
:
: rg
:
But... 100 degrees F is cool...